list-objects-filter: use BUG rather than die
[git] / diffcore-delta.c
1 #include "cache.h"
2 #include "diff.h"
3 #include "diffcore.h"
4
5 /*
6  * Idea here is very simple.
7  *
8  * Almost all data we are interested in are text, but sometimes we have
9  * to deal with binary data.  So we cut them into chunks delimited by
10  * LF byte, or 64-byte sequence, whichever comes first, and hash them.
11  *
12  * For those chunks, if the source buffer has more instances of it
13  * than the destination buffer, that means the difference are the
14  * number of bytes not copied from source to destination.  If the
15  * counts are the same, everything was copied from source to
16  * destination.  If the destination has more, everything was copied,
17  * and destination added more.
18  *
19  * We are doing an approximation so we do not really have to waste
20  * memory by actually storing the sequence.  We just hash them into
21  * somewhere around 2^16 hashbuckets and count the occurrences.
22  */
23
24 /* Wild guess at the initial hash size */
25 #define INITIAL_HASH_SIZE 9
26
27 /* We leave more room in smaller hash but do not let it
28  * grow to have unused hole too much.
29  */
30 #define INITIAL_FREE(sz_log2) ((1<<(sz_log2))*(sz_log2-3)/(sz_log2))
31
32 /* A prime rather carefully chosen between 2^16..2^17, so that
33  * HASHBASE < INITIAL_FREE(17).  We want to keep the maximum hashtable
34  * size under the current 2<<17 maximum, which can hold this many
35  * different values before overflowing to hashtable of size 2<<18.
36  */
37 #define HASHBASE 107927
38
39 struct spanhash {
40         unsigned int hashval;
41         unsigned int cnt;
42 };
43 struct spanhash_top {
44         int alloc_log2;
45         int free;
46         struct spanhash data[FLEX_ARRAY];
47 };
48
49 static struct spanhash_top *spanhash_rehash(struct spanhash_top *orig)
50 {
51         struct spanhash_top *new_spanhash;
52         int i;
53         int osz = 1 << orig->alloc_log2;
54         int sz = osz << 1;
55
56         new_spanhash = xmalloc(st_add(sizeof(*orig),
57                              st_mult(sizeof(struct spanhash), sz)));
58         new_spanhash->alloc_log2 = orig->alloc_log2 + 1;
59         new_spanhash->free = INITIAL_FREE(new_spanhash->alloc_log2);
60         memset(new_spanhash->data, 0, sizeof(struct spanhash) * sz);
61         for (i = 0; i < osz; i++) {
62                 struct spanhash *o = &(orig->data[i]);
63                 int bucket;
64                 if (!o->cnt)
65                         continue;
66                 bucket = o->hashval & (sz - 1);
67                 while (1) {
68                         struct spanhash *h = &(new_spanhash->data[bucket++]);
69                         if (!h->cnt) {
70                                 h->hashval = o->hashval;
71                                 h->cnt = o->cnt;
72                                 new_spanhash->free--;
73                                 break;
74                         }
75                         if (sz <= bucket)
76                                 bucket = 0;
77                 }
78         }
79         free(orig);
80         return new_spanhash;
81 }
82
83 static struct spanhash_top *add_spanhash(struct spanhash_top *top,
84                                          unsigned int hashval, int cnt)
85 {
86         int bucket, lim;
87         struct spanhash *h;
88
89         lim = (1 << top->alloc_log2);
90         bucket = hashval & (lim - 1);
91         while (1) {
92                 h = &(top->data[bucket++]);
93                 if (!h->cnt) {
94                         h->hashval = hashval;
95                         h->cnt = cnt;
96                         top->free--;
97                         if (top->free < 0)
98                                 return spanhash_rehash(top);
99                         return top;
100                 }
101                 if (h->hashval == hashval) {
102                         h->cnt += cnt;
103                         return top;
104                 }
105                 if (lim <= bucket)
106                         bucket = 0;
107         }
108 }
109
110 static int spanhash_cmp(const void *a_, const void *b_)
111 {
112         const struct spanhash *a = a_;
113         const struct spanhash *b = b_;
114
115         /* A count of zero compares at the end.. */
116         if (!a->cnt)
117                 return !b->cnt ? 0 : 1;
118         if (!b->cnt)
119                 return -1;
120         return a->hashval < b->hashval ? -1 :
121                 a->hashval > b->hashval ? 1 : 0;
122 }
123
124 static struct spanhash_top *hash_chars(struct diff_filespec *one)
125 {
126         int i, n;
127         unsigned int accum1, accum2, hashval;
128         struct spanhash_top *hash;
129         unsigned char *buf = one->data;
130         unsigned int sz = one->size;
131         int is_text = !diff_filespec_is_binary(one);
132
133         i = INITIAL_HASH_SIZE;
134         hash = xmalloc(st_add(sizeof(*hash),
135                               st_mult(sizeof(struct spanhash), 1<<i)));
136         hash->alloc_log2 = i;
137         hash->free = INITIAL_FREE(i);
138         memset(hash->data, 0, sizeof(struct spanhash) * (1<<i));
139
140         n = 0;
141         accum1 = accum2 = 0;
142         while (sz) {
143                 unsigned int c = *buf++;
144                 unsigned int old_1 = accum1;
145                 sz--;
146
147                 /* Ignore CR in CRLF sequence if text */
148                 if (is_text && c == '\r' && sz && *buf == '\n')
149                         continue;
150
151                 accum1 = (accum1 << 7) ^ (accum2 >> 25);
152                 accum2 = (accum2 << 7) ^ (old_1 >> 25);
153                 accum1 += c;
154                 if (++n < 64 && c != '\n')
155                         continue;
156                 hashval = (accum1 + accum2 * 0x61) % HASHBASE;
157                 hash = add_spanhash(hash, hashval, n);
158                 n = 0;
159                 accum1 = accum2 = 0;
160         }
161         QSORT(hash->data, 1ul << hash->alloc_log2, spanhash_cmp);
162         return hash;
163 }
164
165 int diffcore_count_changes(struct diff_filespec *src,
166                            struct diff_filespec *dst,
167                            void **src_count_p,
168                            void **dst_count_p,
169                            unsigned long *src_copied,
170                            unsigned long *literal_added)
171 {
172         struct spanhash *s, *d;
173         struct spanhash_top *src_count, *dst_count;
174         unsigned long sc, la;
175
176         src_count = dst_count = NULL;
177         if (src_count_p)
178                 src_count = *src_count_p;
179         if (!src_count) {
180                 src_count = hash_chars(src);
181                 if (src_count_p)
182                         *src_count_p = src_count;
183         }
184         if (dst_count_p)
185                 dst_count = *dst_count_p;
186         if (!dst_count) {
187                 dst_count = hash_chars(dst);
188                 if (dst_count_p)
189                         *dst_count_p = dst_count;
190         }
191         sc = la = 0;
192
193         s = src_count->data;
194         d = dst_count->data;
195         for (;;) {
196                 unsigned dst_cnt, src_cnt;
197                 if (!s->cnt)
198                         break; /* we checked all in src */
199                 while (d->cnt) {
200                         if (d->hashval >= s->hashval)
201                                 break;
202                         la += d->cnt;
203                         d++;
204                 }
205                 src_cnt = s->cnt;
206                 dst_cnt = 0;
207                 if (d->cnt && d->hashval == s->hashval) {
208                         dst_cnt = d->cnt;
209                         d++;
210                 }
211                 if (src_cnt < dst_cnt) {
212                         la += dst_cnt - src_cnt;
213                         sc += src_cnt;
214                 }
215                 else
216                         sc += dst_cnt;
217                 s++;
218         }
219         while (d->cnt) {
220                 la += d->cnt;
221                 d++;
222         }
223
224         if (!src_count_p)
225                 free(src_count);
226         if (!dst_count_p)
227                 free(dst_count);
228         *src_copied = sc;
229         *literal_added = la;
230         return 0;
231 }